It is known to provide an image forming apparatus capable of preventing unauthorized individuals from reading or printing certain documents, e.g., a confidential document. For example, there are image forming apparatuses using copy-forgery-prohibited pattern image processing technology that allow characters to appear in the background of an image when the document is copied. Using this technology, extremely small dots existing on the document disappear when the image is read to embed latent characters such as “COPY” or “DO NOT COPY” in the background of a confidential document. This can be achieved by the use of two areas including a latent image portion formed of a group of small dots and a background portion formed of a group of large dots, thereby allowing those characters to appear when copying the confidential document.
There is also known an image forming apparatus employing a copy guard technology in which a special dot pattern is embedded in the background of the confidential document or the like when outputted. The outputting of the image data is prevented when the special dot pattern is detected when the document image is read at the time of copying thereof.
It is also known to employ a technology combining the above-mentioned two technologies. For example, a technology for forming the background portion (area that does not disappear at the time of the copying) used for a copy-forgery-prohibited pattern image processing with the special dot pattern to which the copy guard technology is applied. This technology allows a copying machine that is capable of detecting the special dot pattern to prevent the illegal copying, as well as, even with respect to a copying machine incapable of detecting the special dot pattern to cause the above-mentioned latent characters to appear on the copy, thereby suppressing the use by a fraudulent third party.